The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Embodiments relate to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing subterranean wells.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone. The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks.
Optimal cement-sheath placement often requires that the cement slurry contain a retarder, a dispersant and a fluid-loss additive. Cement retarders delay the setting of the cement slurry for a period sufficient to allow slurry mixing and slurry placement in the annular region between the casing and the borehole wall, or between the casing and another casing string. Dispersants help maintain the proper rheological properties of the cement slurry, promoting optimal fluid displacement—especially in long, narrow annuli. Fluid-loss additives help prevent the fluid phase of the cement slurry from escaping into the formation, leaving the solids behind.
A wide range of chemical compounds may be employed as cement retarders. The most common classes include lignosulfonates, cellulose derivatives, hydroxycarboxylic acids, saccharide compounds, organophosphonates and certain inorganic compounds such as sodium chloride (in high concentrations) and zinc oxide. A more complete discussion of retarders for well cements may be found in the following publication—Nelson E B, Michaux M and Drochon B: “Cement Additives and Mechanisms of Action,” in Nelson E B and Guillot D. (eds.): Well Cementing (2nd Edition), Schlumberger, Houston (2006) 49-91.
Certain types of retarders may be blended with other compounds to extend their useful temperature range, improve cement-slurry properties, or both. For example, the useful temperature range of certain lignosulfonate retarders may be extended to more than 260° C. by adding sodium tetraborate decahydrate (borax). Sodium gluconate may be blended with a lignosulfonate and tartaric acid to improve the rheological properties of the cement slurry. The useful temperature range of organophosphonate retarders may also be extended to more than 260° C. by adding borate compounds. For well cementing, the most common dispersants are generally sulfonated aromatic polymers such as polynaphthalene sulfonate, polymelamine sulfonate and polystyrene sulfonate. Fluid-loss additives for well cements include water-soluble polymers such as polysaccharides (e.g., hydroxyethylcellulose), polyamines, polyvinylalcohols, and polyacrylates. Particulates such as bentonite, crosslinked polyvinylalcohols and latexes are also common. Thus, a myriad of retarders, retarder blends, dispersants and fluid-loss additives exist which may be applicable to a wide range of subterranean-well conditions.
When cementing high-pressure, high-temperature (HPHT) wells, the cement-slurry design may be complex, involving several additives that must be mutually compatible in order to achieve a successful cement job. In general, the well-cementing industry considers HPHT wells to begin at 150° C. (300° F.) bottomhole temperature and 69 MPa (10,000 psi) bottomhole pressure. The additives must remain stable at temperatures that may exceed 260° C. for a period sufficient to at least allow proper cement-slurry placement. Additive decomposition during placement may have undesirable consequences, including slurry gelation (strong viscosity increase) and premature setting. Similarly, reactions between additives may also cause rheological difficulties.
Under HPHT conditions, undesirable interactions between the additives and the cement become more likely. Such interactions may, in some cases, result in shorter thickening times, compromised performance of some additives (e.g. fluid-loss-control agents) and gelation problems (often referred to as a “quaternary gel”). The severity of such problems is strongly cement dependent.
Despite the valuable contributions of the prior art, there remains a need for means preventing gelation, premature setting, or both in Portland-cement slurries at temperatures up to and exceeding 260° C.